Unable To Add Two Strings To A Hash Map

I am trying to add two strings to a hash map. the first being the key, a 3 digit code that can have duplicates, and the value to store in an ArrayList. From what I've read, when add a key to the hashmap that is a duplicate, the previous gets overwritten and that is why I am trying to put the values in an array list. I was hoping that when the key is looked up, it would print all the values associated with that key:

class library{
HashMap<String, ArrayList<String>> checkoutBooks =
new HashMap<String, ArrayList<String>>() ;
ArrayList<String> patName = new ArrayList<String>() ;
public void checkoutBook(String isbn, String patron) {
patName.add(patron) ;

[Code]...

So when I try to add values such as:

library.checkoutBook("000", "test 1");
library.checkoutBook("000", "test 2");
library.checkoutBook("001", "test 3");
library.checkoutBook("001", "test 4");
library.checkoutBook("002", "test 5");
library.checkoutBook("002", "test 6");

[Code]...